The present invention relates generally to light source driver circuits. More particularly, the invention relates to driver circuits for LED lighting having auxiliary circuitry to protect against output short conditions.
Light emitting diode (LED) lighting is growing in popularity due to decreasing costs and long life compared to incandescent lighting and fluorescent lighting. LED lighting can also be dimmed without impairing the useful life of the LED light source.
Isolated constant current source topologies that are commonly used in light source (e.g., LED) driver circuits include flyback converters, forward converters, LLC converters, and half-bridge isolated buck converters. Flyback converters and forward converters have low efficiency and require the use of high-voltage MOSFET devices. LLC converters have insufficient output voltage range, and the output is not self-limiting. Half-bridge isolated buck converters require hard switching of the MOSFET devices, have low efficiency, require complicated controllers, and the output is not self-limiting.
There is a particular CLASS-2 LED driver as defined in Underwriters Laboratories specifications. The specification requires the LED driver to be isolated and have a maximum output voltage of 60 volts DC at any given time. If a maximum voltage of a load (e.g., a light source such as an LED string) is close to 60 volts, then limiting the output voltage to 60 volts by operation of the control loop in the driver circuit is difficult because control loops inherently have over-shoot and delay.
An LED driver circuit should be capable of driving different loads that have different numbers of LEDs. Thus, the LED driver has to be capable of a wide range of output voltages while maintaining control of the output current.
Also, the maximum output current from an LED driver circuit must always be less than 8 A, even where a component has failed within the driver circuit. One particularly bad condition is where a component has failed and caused the output wires to be shorted. A reliable driver circuit should be designed to control the output current tightly enough to meet the UL CLASS-2 standard even in this case.